Endophthalmitis is a sight-threatening infection of the interior of the eye resulting from the introduction of organisms into the posterior segment. Severe endophthalmitis often results in irreversible damage to delicate cells of the retina and explosive intraocular inflammation, ultimately resulting in vision loss. Despite aggressive antibiotic, anti-inflammatory, and surgical treatment, significant vision (if not the eye itself) can be lost in a short period of time. The regularity of treatment failures highlights the need for better therapeutics which target the pathogenic mechanisms of disease. Current treatments can sterilize the eye, but fail at limiting damaging inflammation and ignore bacterial products which can be equally as damaging. Our research has focused on the pathogenic mechanisms of Bacillus post-traumatic endophthalmitis (PTE) and, more recently, on new models of Klebsiella and Staphylococcus endogenous endophthalmitis (EE). We have identified a theme common to endophthalmitis caused by these and other pathogens: retinal vascular permeability is an initial and important component of the disease process. This new proposal focuses on vascular permeability during the very early stages in the evolution of bacterial endophthalmitis, when organisms enter the eye and inflammation is initiated. We hypothesize that vascular permeability triggered in PTE and post-operative (POE) bacterial endophthalmitis facilitates influx of inflammatory cells into the posterior segment (Aim 1). We similarly hypothesize that vascular permeability initiated during systemic infection facilitates the entry of organisms into the eye, resulting in EE (Aim 2). In these types of endophthalmitis, the outcome is infection, inflammation, and the potential for vision loss. The underlying mechanisms of retinal vascular permeability leading to cellular and bacterial access into the eye during these different types of endophthalmitis are not clear. However, if these events include identifiable common factors that can be therapeutically targeted, infection and inflammation could be limited, resulting in preservation of vision. The aims to be tested are built upon published and new data in our models of bacterial PTE/POE and new models of bacterial EE, with the goal of deciphering the mechanisms of retinal vascular permeability during endophthalmitis. Current therapies do not consider changes in ocular barrier integrity, a likely precursor event to inflammation in PTE/POE and infection in EE. Published studies on endophthalmitis by us and other groups have not focused on the very early stages of bacterial recognition in the eye prior to overt inflammation, a critical time when the cascades responsible for damaging inflammation could be therapeutically averted. Therefore, the findings from this proposal will provide important and clinically relevant information regarding ocular barrier defense during intraocular infection and identify potential targets for therapeutic intervention that will prevent devastating and blinding effects of the disease.